Due to the high data rates in recently developed communication systems having data transmission rates, for example, 25 Gbps, signal integrity, such as, for instance, the reduction of cross-talk between signal lines, has become a major concern.
Ideally, an interconnection system will carry signals without distortion. One type of distortion is called cross-talk. Cross-talk occurs when one signal creates an unwanted signal on another signal line. Generally, cross-talk is caused by electromagnetic coupling between signal lines. Therefore, cross-talk is a particular problem for high-speed, high-density interconnection systems. Electromagnetic coupling increases when signal lines are closer together or when the signals they carry are of a higher frequency. Both of these conditions are present in a high-speed, high-density interconnection system. Discontinuities in the connector often exacerbate any cross-talk problems. It is generally known to insert a shield into the connectors in the connection system in order to reduce the impact of cross-talk.
Furthermore, it is known to use differential signals for transporting information. One differential signal is carried on two conductors, with the signal being represented as the difference in electrical levels between the conductors. A differential signal is more resistant to cross-talk than a single-ended signal, because any stray signals impinging on the conductors will generally change the level on both conductors, but do not alter the difference in levels.
Consequently, conventional high-speed transmission assemblies use circuit boards as a substrate to be connected to another board having a pair of wires for carrying each differential signal. A first printed circuit board has traces and pads on at least one of its surfaces, wherein particular contact pads are to be contacted by being soldered to mating contact pads on a second printed circuit board. The traces transmit electrical signals across the respective first and second printed circuit boards.
The transition from the first printed circuit board to the second printed circuit board of course needs to be capable of handling the high data rates as well. Moreover, due to the small form factor of the interconnect, cross-talk between adjacent or even further remote pairs of lines is an important parameter. Ground plane layers are typically used for shielding the lines against signal distortions.
However, at the electrical connection interface where two printed circuit boards are soldered, impedance compensation has to be used in order avoid distortions of the signal due to signal reflection. Therefore, conventional electrical connection interfaces dispense with internal ground layers in the particular region where the conductive pads of the traces are soldered to each other. An electrical connection between the ground plane layers of the first and second printed circuit board is established by providing ground-to-ground interconnecting leads that extend over the ground plane free gap. The size of such a board-to-board interconnect can be rather significant, typically 1200 μm per differential pair of signals, considering current reflow soldering technology allowing minimum 200 μm spacing between solder balls and restricting solder ball width to 200 μm.
Thereby, however, cross-talk between the signal lines and other effects degrading the signal integrity are no longer eliminated effectively enough.